Go to the url address: about:config. This should load the Firefox config settings in the window for editing. You may see a warning message stating, "This might void your warranty!" that will require you to click through to load the settings. Then find the browser.cache.check_doc_frequency key and change it to 1.

You can also clear the entire cache if you want to. Just go to Options ► Advanced ► Network ► Cached Web Content ► Clear Now. Then click the usual refresh button or hit F5, and the page will be fully updated!